A magnetic resonance imaging (MRI) study of changes in the angle between the tracheal and laryngeal axes in the sniffing, neutral and extended head and neck positions

Abstract

Background; Although the sniffing position has historically been used for conventional laryngoscopy, the ideal position for videolaryngoscopy is yet to be elucidated. When a hyperangulated blade is utilised in videolaryngoscopy, despite improved glottic visualisation, the acute angle between the laryngeal and tracheal axes results in the inability to advance the tube into the trachea. We hypothesise that the position that produces the smallest angle between the laryngeal and tracheal axes would most likely facilitate passage of a tube into the trachea during videolaryngoscopy using a hyperangulated blade. Methods20 healthy young adult volunteers underwent magnetic resonance imaging scanning in three anatomic positions: head in the neutral position, in the sniffing position and in the extended position. In total 60 T2-weighted sagittal magnetic resonance images were obtained and the following measurements were made: angle A (the angle between the laryngeal and tracheal axes), angle B (the angle between the laryngeal and pharyngeal axes), angle C (the angle between the pharyngeal and oral axes), angle D (the angle between the laryngeal axis and line of vision) and angle E (the angle between the line of vision and pharyngeal axis). ResultsMean (SD) angle A (in ◦) in neutral, sniffing and extended positions were 20.7 (6.8), 10.8 (6.9) and 30.5 (5.2) with p < 0.001. Angle A and C were significantly reduced when the head position was shifted from neutral to sniffing position. Angle D was smallest in the extended position. The difference in all five angles in three positions was statistically significant (p < 0.001). ConclusionAngle A was smallest in the sniffing position, therefore this position is most likely produce a better alignment between laryngeal and tracheal axes.